1. Field of the Invention
The present invention relates generally to electronic components and, more particularly, to surface mount-type electronic components.
2. Description of the Related Art
A surface mount-type electronic component previously developed by the present inventor is a piezoelectric component illustrated in FIG. 1. This piezoelectric component is a capacitor-integrating piezoelectric oscillator having one oscillator R and two capacitors C.sub.1, and C.sub.2 for use in a Colpitts oscillator circuit. The electric circuit of this component is shown in FIG. 2.
Referring to FIG. 1, this piezoelectric component includes an insulating substrate 20, an oscillator device 40, a capacitor device 50, and a metallic cap 60. The substrate 20 is a substantially rectangular thin sheet preferably made from alumina ceramics, and three band-like electrodes 22, 23 and 24 are preferably provided on the upper surface of the substrate 20 and arranged substantially parallel to the widthwise direction of the substrate 20. The electrodes 22, 23 and 24 serve as an input electrode, a ground electrode and an output electrode, respectively. Both ends of the electrodes 22 through 24 respectively extend to recesses or through-holes 20a formed on the side edge surfaces of the substrate 20 and are connected to the electrodes disposed on the lower surface of the substrate 20 via the electrodes formed on the inner surfaces of the recesses 20a.
A frame-shaped insulating layer 25 preferably made from a resin base paste or a glass base paste is provided on the top portion of the substrate 20 on which the cap 60 is bonded. The thickness of the insulating layer 25 is desirably determined to be within a range, for example, from about 20 to 40 .mu.m, in order to reduce a level difference or difference in vertical height or location created by the electrodes and also to sufficiently insulate the electrodes from the cap 60 which will be explained in greater detail below. The insulating layer 25 is baked or cured after it has been printed.
Bonded onto the substrate 20 is a laminated block formed by integrating the oscillator device 40 and the capacitor device 50 via materials 30 through 32 which have both conducting and adhering functions, such as a conductive adhesive.
The oscillator device 40 is a thickness shear vibration-mode device in which an electrode 42 is disposed to cover an area beginning at a first end of the device 40 and extending about two thirds along a length of a top surface of a piezoelectric ceramic substrate 41 while an electrode 43 is disposed to cover an area beginning at a second end of the device 40 and extending approximately two thirds of a length of a bottom surface of the substrate 41. The inner ends of the electrodes 42 and 43 overlap each other at the intermediate portion of the device 40 across the piezoelectric substrate 41 so as to form a vibrating section. The outer ends 42a and 43a of the electrodes 42 and 43 are routed in a roundabout manner to the bottom surface of the piezoelectric substrate 41 via the respective edge surfaces of the substrate 41.
Moreover, the capacitor device 50 has a dielectric substrate 51 (for example, a ceramic substrate) preferably having the same length and width as the oscillator device 40. Two individual electrodes 52 and 53 are disposed on a top surface of the dielectric substrate 51 so as to extend from both ends towards a center portion of the substrate 51, while one opposing electrode 54 is provided on a bottom surface of the substrate 51 to opposedly face the individual electrodes 52 and 53. Two capacitors C.sub.1, and C.sub.2 are formed in the opposing portion between the opposing electrode 54 and the individual electrodes 52 and 53. The outer ends 52a and 53a of the electrodes 52 and 53 are routed in a roundabout fashion to the bottom surface of the substrate 51 via the respective edge surfaces of the substrate 51.
The bottom surface of the oscillator device 40 and the top surface of the capacitor device So are bonded at both ends via materials 33 and 34 having-both conducting and adhering functions, such as a conductive adhesive. Simultaneously, a space for vibration can be formed between the vibrating section of the oscillator device 40 and the capacitor device 50 by virtue of the thickness of the materials 33 and 34. In this manner, one electrode 42 of the oscillator device 40 and one individual electrode 52 of the capacitor device 50 are connected, while the other electrode 43 of the oscillator device 40 and the other individual electrode 53 of the capacitor device 50 are coupled. It should be noted that resin-containing, frequency-adjusting damping materials 35 and 36 are applied onto both ends of the top surface of the oscillator device 40.
After the oscillator device 40 and the capacitor device 50 are integrally attached to each other, the bottom surface of the capacitor device 50 is bonded onto the substrate 20 via materials 30 through 32. Then, the outer end 52a of one individual electrode 52 of the capacitor device 52 is connected to the input electrode 22; the outer end 53a of the other individual electrode 53 is coupled to the output electrode 24; and the opposing electrode 54 is connected to the ground electrode 23.
The cap 60 is fixed at its opening onto the substrate 20 with an adhesive 61 to cover the oscillator device 40 and the capacitor device 50. The adhesive 61, which is preferably an epoxy-type adhesive, is applied to the bottom surface of the opening of the cap 60 which is then bonded to the insulating layer 25. Then, the adhesive 61 is cured.
In the piezoelectric component described above using the metallic cap 60, it may be preferred to that the cap 60 is electrically grounded in order to inhibit stray capacitance and to shield electric noise entering from the exterior.
As one of the methods for grounding the cap 60, the following technique is available. After the cap 60 is attached to the substrate 20, a conductive paste 70 is applied with a dispenser, as shown in FIG. 3, to connect the outer surface of the cap 60 to the ground electrode 23 of the substrate 20, thereby establishing an electrical connection between the cap 60 and the ground electrode 23.
However, this method presents the following problems. The application of the conductive paste 70 with a dispenser is time-consuming, and also, additional work, i.e., the application of the paste 70, is required after the piezoelectric component has been completed, thereby lowering productivity. Further, the adhesive 61 forms a fillet or bump seen in FIGS. 4 and 5 on the exterior of the cap 60 to bond the cap 60 to the substrate 20, and it is necessary that the conductive paste 70 be applied to a position or vertical level higher than the fillet in order to electrically connect the cap 60 to the ground electrode 23. It is very difficult, however, to apply the paste 70 to a position higher than the fillet while preventing the paste 70 from flowing into the recessed portion 20a of the substrate 20. In practice, the conductive paste 70 flows into the recessed portion 20a, as illustrated in FIG. 4, to disadvantageously lower solderability and create a poor electrical connection between the cap 60 and the substrate 20, as shown in FIG. 5.